The present invention relates to fluids useful for subterranean operations, and more particularly, to novel treatment fluids and methods for the removal of filter cake in subterranean formations.
When well bores are drilled into subterranean formations, drilling fluids are used which will minimize damage to the permeability of the formations and their ability to produce hydrocarbons. Servicing fluids may be used when completion operations are conducted in producing formations or when conducting work-over operations in the formations. The drilling and servicing fluids may deposit a layer of particles known as “filter cake” on the walls of the well bores within the producing formations. The filter cake, among other things, may prevent the drilling and servicing fluids from being lost into the formations and/or limit drill solids from entering the porosities of the producing formations.
Well bores drilled in certain subterranean formations are sometimes completed as open holes, i.e., without a casing or liner installed therein. Special drilling fluids referred to in the art as “drill-in fluids” may be used to drill such well bores, among other reasons, to minimize the damage to the permeability of the producing zones or formations. The drill-in fluid may form a filter cake on the walls of the well bore, which may prevent or reduce fluid loss during drilling, and upon completion of the drilling, may stabilize the well bore during subsequent completion operations such as placing a gravel pack in the well bore.
After completion operations in the well bore have been completed, the filter cake remaining on the walls of the well bore must be removed. This can be accomplished, among other ways, by contacting the filter cake with an aqueous acid solution. However, the use of an aqueous acid solution may be hazardous to personnel or may cause corrosion on surfaces and/or equipment in the well bore. Also, the aqueous acid solution may react rapidly at the initial point of contact with the well bore to create a fluid loss zone into which the rest of the acid is lost, leaving much of the filter cake untreated and in place.
As an alternative method, acid generating compounds have been employed for filter cake removal in place of aqueous acid solutions. Acid generating compounds produce acid over time and thus may be less hazardous to personnel. Because of this time-dependent release of acid, these compounds also may be able to flow further into the well bore as they react with the filter cake or completely across the interval of interest, removing a larger amount of the filter cake and reducing the creation of fluid loss zones.
Such treatments with acid generating compounds may be performed independent of other processes, or they may be performed during a completion operation, such as the installation of a sand screen and/or gravel pack. The inclusion of these compounds in fluids for completion operations may provide a more time- and cost-effective well completion procedure. However, since the exact field conditions and make up of the drill-in fluid filter cake may not be known precisely beforehand, the break may not be accomplished at the planned time. The inclusion of these compounds in completion fluids could then result in premature removal of portions of the filter cake, which may result in a loss of the completion fluid into the surrounding portions of the subterranean formation. Treatments with these compounds also may require a substantial amount of testing in order to determine the proper amount of such compounds to most effectively produce the desired time-dependent filter cake removal.